Abrupt change of optical cross-section easily occurs on the periphery of a light guide plate (LGP) positioning structure of a backlight, then, an abrupt change of optical effect occurs, so that a shadow is generated at a point far away from a light source, and a bright area is generated at a position near the light source. Currently, to solve the problem, a common way is to arrange a reflector plate on the periphery of the LGP positioning structure to balance the bright and dark shadows caused by abrupt change of edge. However, the reflector plate is easy to fall off and is difficult to adjust the shadow problem.